Fix port syntax in host header of HTTP requests.
Fix port syntax in host header of HTTP requests.

When constructing a host header, if PORT is an integer, add it to the
header, otherwise omit it.

The previous behavior resulted in host headers like
"www.example.com:NIL", which failed on S3 but (confusingly) worked on
CloudFront.

daewok/quicklisp-client,tamurashingo/quicklisp-client,quicklisp/quicklisp-client